High-pressure sodium lamps are well known in the art and are widely used for street, roadway and other outdoor lighting applications. A high-pressure sodium lamp typically consists of a cylindrical transparent or translucent arc tube which contains pressurized sodium vapor.
The arc tube generally has a pair of electrodes therein, and a current flows through the sodium vapor in the arc tube to excite the sodium atoms. The current is preferably an ac current, which typically offers an increased service life relative to a dc current. The energy which is given off by the excitation and relaxation of the sodium ions is converted into visible light and heat.
The arc tube is generally enclosed in a glass bulb or similar outer jacket to isolate the arc tube from the environment, thereby preventing oxidation of the electrodes and other metallic parts, stabilizing the operating temperature of the lamp and significantly reducing any ultraviolet radiation emitted by the excitation of the sodium ions.
In the art of illumination, the color temperature refers to the absolute temperature (in degrees Kelvin) of a blackbody radiator whose chromaticity most nearly resembles that of the light source.
As appreciated by those skilled in the art, the color temperature of a high-pressure sodium lamp is a function of the peak current through the lamp. The color temperature determines the hue of the light produced by the lamp, commonly referred to as lamp color. It is considered important in the art to maintain a desired peak current so that the lamp will have a desired lamp color.
Peak current through the lamp is a function of the lamp's internal impedance. One of the problems associated with the operation of high-pressure sodium lamps is that the impedance of the lamp varies over time, both due to internal temperature effects, as well as due to the deterioration of the lamp over its service life.
Additionally, variations in lamp impedance exist from one lamp to another due to manufacturing tolerances, whether from the same manufacturer or from one manufacturer to another.
Thus, the internal impedance of a lamp will vary over time, and the internal impedance of any replacement lamp will also vary, relative to the internal impedance of the initial lamp. Accordingly, it has heretofore been difficult to maintain a constant peak current through a lamp given the fluctuation in lamp impedance and hence maintain a substantially uniform lamp color.